calculate some statistics/aggregations for each date using a certain amount of previous dates

puhoma

Hello. I have dataset with dates. I want to calculate some statistics/aggregations for each date using a certain amount of previous dates ( for example, to calculate for each day a sum of sales of 15 previous dates). What is the best way to do it. I know some clumsy ways, but think it could be better ones.

puhoma

And I would like to use it not only with dates. Sometimes I need to calculate statistics using certain amount of previous raws  for each raw (that were sorted in a special way before).